1. Field of the Invention
This invention is broadly concerned with a so-called hybrid vehicle which has a combined power source consisting of an internal-combustion engine, such as a gasoline or diesel engine, and an electric motor driven by electrical energy stored in a battery.
2. Description of the Related Art
Generally, in the conventional vehicle, the rotary power of an internal combustion engine, using gasoline for fuel, is transmitted to front and/or rear driving wheels through an automatic or a manual transmission unit. The internal combustion engine burns a compressed gas mixture of air and gasoline and outputs the energy of combustion as the engine torque. Therefore, the conventional vehicle has noise problems caused by combustion and environmental pollution problems caused by exhaust gas.
In consideration of these problems, various electric vehicles, whose energy source is electricity instead of gasoline, have been developed. Electric vehicles known to the public carry on board a battery which stores electricity as an energy source and an electric motor for driving the wheels. Therefore, the running of the electric vehicle is not accompanied by noise or by exhaust gas.
However, at present, the capacity of the battery is not necessarily sufficient and, as a result, the range of the car tends to be shorter than demanded. When it is necessary to secure sufficient range using the electric motor for driving the wheels, use of a big battery which is inconvenient from the viewpoints of weight and space is inevitable. Also, because of restriction on the size of the electric motor, the driving force obtained with the electric motor is weaker than that of the vehicle using an internal combustion engine. Therefore, a rapid start, running under heavy load, and high-speed running are difficult to be realized.
Based on the problems mentioned above, so-called hybrid vehicles, which use both an internal combustion engine and an electric motor, have been proposed. The hybrid vehicles include, for example: (1) a series type hybrid vehicle wherein electric energy is generated by using a generator driven by an internal combustion engine and an electric motor utilizes this electric energy as vehicle driving power (see Japanese Patent Application Laid-open No. 62-104403); and (2) a parallel type hybrid vehicle which uses both the internal combustion engine and the electric motor, arranged in parallel, as drive sources (see Japanese Patent Application Laid-open No. 59-63901 and U.S. Pat. No. 4,533,011).
These latter hybrid vehicles, which have been known to the public, use the electric motor as the main source of driving power during low or middle speed running in cities and use the internal combustion engine which has more power during high speed running in the suburbs. The truth of the matter is that the above-mentioned conventional hybrid vehicles cannot run very fast during driving in cities. In the suburbs, it can be said that efficient running which takes advantage of both the internal combustion engine and the electric motor has not been well considered.
The purpose of this invention is to solve the various problems which exist in the conventional hybrid vehicles and not only to prevent noise and exhaust gas, but also to generate sufficiently strong driving force for city travel. For suburban driving, it provides a hybrid vehicle which can effectively use both the internal combustion engine and the electric motor in consideration of energy efficiency.